When the cooling cycle of a cooling system is overcharged with a refrigerant, the surplus gas is compressed by a compressor, so that an abnormal high pressure is built up in the cycle to give an ill effect to the pressure resistance of the cycle parts. This may also cause improper cooling or an increased power requirement. Further, in such a situation, liquid compression would be done by the compressor to invite damage to the compressor valves. Devices for avoiding such trouble, for instance, refrigerant overcharge detectors comprising a liquid level sensor disposed at an upper part of a receiver tank and designed to detect the overcharge from the output of said sensor are disclosed in Japanese Utility Model Publication No. 14303/72, Japanese Utility Model Laid-Open No. 130175/80 and other literatures.
However, the results of the experiments conducted by the present inventors on said disclosed devices clarified that these devices with said simple setup tend to give rise to the problems such as mentioned below.
Firstly, in these conventional devices, as seen from FIG. 1 where the rotational speed of the compressor drive system is plotted as abscissa and the liquid level in a receiver tank as ordinate, the liquid level in the receiver tank rises up in accordance as the compressor speed increases, and when the detection is made at a compressor speed above a certain specified value, an overcharge detection signal could be issued even if the refrigerant charge is normal. This is due to the following reason: when the speed of the compressor drive system elevates, the amount of air received by the condenser is accordingly increased to cause a rise of its capacity, admitting a greater amount of refrigerant into the receiver tank.
Also, as seen from FIG. 2 where the compressor operation time is plotted as abscissa and the liquid level in the receiver tank as ordinate, the liquid level sharply rises up immediately after start of the compressor and its peak is expressed as a transient response and it could be erroneously detected as an overcharge. This happens because during the period immediately after start of the compressor, the liquid refrigerant which has been stored in the condensor while the compressor was inoperative is caused to rush into the receiver tank.